This invention relates to a hot gas engine and, in particular, to controlling the operation of a Stirling engine suitable for use in an automotive application.
The Stirling or hot gas engine cycle is well known in the art. A two cylinder engine is described in U.S. Pat. Nos. 3,984,983 and 3,999,388 while the operation of a four cylinder engine is further described in U.S. Pat. Nos. 3,914,940 and 4,474,003. The Stirling engine is durable, clean burning and exhibits relatively high efficiency when compared to the more conventional internal combustion engine. The Stirling engine, however, is relatively slow to respond to changes in power demands and thus difficult to adapt for use in motor vehicles where engine acceleration and deceleration must be rapid. Recently efforts have been undertaken to improve the response time of the Stirling engine so that it might be better adapted to use in motor vehicles.
In the hot gas engine, the engine power output is regulated by changing the pressure of a working gas, such as helium or hydrogen, contained within the engine. To increase the engine's output power, the internal gas pressure is increased by adding gas to the engine from an external supply reservoir. To decrease the engine pressure, gas is typically pumped from the engine back to the supply reservoir using a compressor.
Single acting or double acting compressors are generally used to pump down the hot gas engine. In either case, the compressor has a single capacity. In order to attain a satisfactory idle pressure, which is usually about four megapascals, the capacity of the single capacity compressor must be relatively low. As a consequence, the pump down rate of the engine is correspondingly slow and the time required to bring the engine pressure from some high operating value to idle is longer than desired particularly when the engine is employed in an automotive application. That portion of the engine gas that cannot be pumped by the limited capacity compressor is short circuited back to the engine. The energy contained in the short circuited gas is dissipated in the engine and represents lost power thereby reducing engine efficiency. This type of efficiency penalty can be relatively large and can only be minimized by increasing the pump down rate.
In U.S. Pat. No. 3,782,119 there is disclosed a hot gas engine employing the Stirling power cycle in which the compressor is replaced by a series of supply tanks. The tanks are maintained at different pressures and, through use of a control valve, one or more of the tanks can be connected to the engine to raise the engine pressure to some desired level. During pressure reduction, engine gas is bled back into tanks again by selectively sequencing the control valves. The valving scheme, by necessity, must be rather complex and maintaining close control over the tank pressures is sometimes difficult. The response of the engine without the aid of a compressor is relatively slow.